The invention relates to a circuit arrangement for operating a discharge lamp, which discharge lamp is connected in a commutation circuit such that a comparatively low-frequency square-wave supply voltage with a comparatively low or medium amplitude is supplied to the lamp in the normal operational phase of the lamp, in which an arc discharge takes place continuously in the lamp, whereas a comparatively high-frequency square-wave supply voltage with a comparatively high amplitude is supplied to the lamp in an ignition phase preceding said normal operational phase.
Such a circuit arrangement is known from U.S. Pat. No. 5,932,976.
More in particular, the invention relates to a phase lying between the ignition phase and said normal operational phase, when no reliable, permanent ignition of the lamp has taken place yet. This phase will be denoted the take-over phase for short hereinafter. In this take-over phase, a comparatively strong charging/discharging current will flow through a capacitor connected in a usual manner in parallel to the discharge lamp, which current constitutes a major load on the supply circuit, which supply circuit in its turn supplies the commutation circuit, and on the switching transistors of the commutation circuit.
U.S. Pat. No. 5,962,981 discloses a circuit arrangement for a discharge lamp, in which patent document the problem is addressed of excessive charging and discharging currents of the lamp capacitor which occur in a run-up phase prior to the normal operational phase, but not of the load formed thereby on the supply circuit which in its turn supplies the commutation circuit.
The invention has for its object to provide a circuit arrangement of the kind mentioned above in which a reduced load on the supply source supplying the commutation circuit is obtained in the take-over phase, so that a higher voltage is maintained than would otherwise be the case at the output of said supply source, which usually has a capacitor connected in parallel which is to supply all power for the ignition circuit and the lamp, so that it is better able to supply the necessary power for heating up the electrodes of the lamp by means of glow discharges and for subsequently igniting a discharge arc in the lamp.
According to the invention, the object is achieved by means of a circuit arrangement for operating a discharge lamp, which discharge lamp is connected in a commutation circuit such that a comparatively low-frequency square-wave supply voltage with a comparatively low or medium amplitude is supplied to the lamp in the normal operational phase of the lamp, in which an arc discharge takes place continuously in the lamp, whereas a comparatively high-frequency square-wave supply voltage with a comparatively high amplitude is supplied to the lamp in an ignition phase preceding said normal operational phase, and a supply source is connected to the commutation circuit, which supply source in its turn supplies the commutation circuit and which supply source is characterized in that, in a take-over phase between said ignition phase and said operational phase, the comparatively low-frequency square-wave supply voltage is suppressed in the initial portion of each half cycle thereof, such that the duty cycle of each wave is reduced and instead of this comparatively low-frequency square-wave supply voltage a comparatively high-frequency square-wave supply voltage is fed to the lamp.
According to the cited U.S. Pat. No. 5,962,981, it is true that a high-frequency switching of the switching transistors of the commutation circuit takes place in an initial period of the low-frequency switching, but no comparatively high-frequency square-wave supply voltage is fed to the lamp as a result of this; on the contrary, a comparatively low-frequency square-wave supply voltage/current is still fed to the lamp during the high-frequency switching of the switching transistors (FIGS. 18, 20, 22).
According to the invention, therefore, a comparatively high-frequency square-wave supply voltage is fed to the lamp in the initial portion of each half cycle of the comparatively low-frequency square-wave supply voltage, in which the square-wave is suppressed and the remaining portion of the half cycle accordingly has a duration which is shorter than that of half a cycle of the low-frequency supply voltage, or of which the duty cycle is below 100% in relation to the half cycle.
The xe2x80x9ccomparatively high frequencyxe2x80x9d is understood to be, for example, a frequency of 50 kHz or even 200 kHz, as against the frequency of 90 Hz in the normal operational phase in which the lamp is xe2x80x9conxe2x80x9d.
A result of the above measure according to the invention is that, as was found in practice, glow discharges arise in the lamp at a low current level in the take-over phase while the comparatively high-frequency square-wave supply voltage is being applied to the lamp, and that ending of the comparatively high-frequency square-wave supply voltage and the actual start of the next (shortened) half cycle of the comparatively low-frequency square-wave supply voltage leads to a current pulse load on the supply source supplying the commutation circuit which is substantially smaller (3 A against 13 A in a practical case) than that which would occur without the measure according to the invention. Not only does this decrease the load on said supply source, with the result that the output capacitor thereof can charge up to a higher voltage, but the current load on the switching transistors of the commutation circuit is also reduced, so that lighter transistors can be used.
The initial portion of each half cycle of the comparatively low-frequency square-wave supply voltage, in which the relevant square wave voltage is suppressed and the comparatively high-frequency square-wave supply voltage is present in its place, may have a duration of 100% in the beginning down to 0% at the end of the take-over phase, percentages relating to the duration of one block of the comparatively low-frequency supply voltage or the half cycle of the frequency thereof, so that in this way a gradual decrease in the high-frequency commutation of the commutation circuit and a gradual increase of the low-frequency commutation take place in an interrelated manner over a duration which may be set empirically to the most effective value.
The invention will now be explained in more detail with reference to the drawing, in which: